1. Field of the Invention
The present invention relates to a wired circuit board and a method of producing the same, more particularly, to a wired circuit board including, for example, a suspension board with circuit and the like and a producing method thereof.
2. Description of the Prior Art
There has been conventionally known a suspension board with circuit including a metal supporting board made of stainless steel, an insulating layer made of a resin, and a conductive pattern made of copper, which are formed successively on the metal supporting board.
In such a suspension board with circuit, a metal supporting board is made of stainless steel, so that a transmission loss is increased in a conductive pattern.
To reduce the transmission loss, it has been proposed that a lower conductor made of copper or a copper alloy containing mainly copper is formed on a suspension made of stainless steel and then an insulating layer, a conductor on the recording side and a conductor on the playback side are successively formed on the lower conductor (see, e.g., Japanese Unexamined Patent Publication No. 2005-11387).
However, in the proposal mentioned above, the insulating layer is formed directly on the lower conductor so that an ion migration phenomenon occurs in which the copper or the copper alloy containing mainly copper of the lower conductor migrates to a surface of the insulating layer or to the inner portion thereof with moisture or water absorption by the insulating layer in the presence of an electric current or voltage. This occasionally causes defective adhesion between the lower conductor and the insulating layer or defective conductivity of the conductors on the recording and playback sides.
When the insulating layer is formed on the lower conductor in the form of a pattern by exposing the photosensitive synthetic resin to light and developing it, an exposure mask must be disposed precisely on the upper portion of the lower conductor, thereby requiring a positioning mark provided on the suspension board.
In order to precisely detect the positioning mark, since it is optically detected, an optically distinguishable contrast is required between the surface of the positioning mark and the surface around the positioning mark.